<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    .active{
      background-color: pink;
    }
  </style>
</head>
<body>
<button class="active">1</button>
<button>2</button>
<button>3</button>
<button>4</button>
<button>5</button>
<script>
  const buttons=document.querySelectorAll("button")
  for (let i = 0; i <buttons.length ; i++) {
    buttons[i].addEventListener("click",function () {
     document.querySelector(".active").className=""
      this.className="active"
    })
  }
</script>
</body>
</html>